This invention relates to classifying or sorting objects, for instance for sorting ore from associated mineral material or sorting diamonds from gangue in the diamond mining industry.
Diamonds can be mined from deposits in rock and in this process it is necessary to crush the rock and then sort the diamonds from the gangue or unwanted material. A process for achieving a primary sort of gangue from diamonds is disclosed in European Patent Application No. EP-A-0 345 949 filed by the Applicants. In this process mixed diamonds and gangue are fed on a broad belt feeder, a narrow transverse strip of which is irradiated by X-rays or visible radiation, the fluorescence or Raman emission of diamond being detected to give a signal and locate the diamond. This signal actuates means for selecting diamonds from the stream of mixed material and directing them to a bin to give a diamond rich concentrate. However, other non-diamond mineral particles also fluoresce under the X-ray irradiation or exhibit broad band luminescence stimulated by the excitation source, e.g. a laser operating in the UV/visible portion of the spectrum. This laser excited broad band luminescence covers the region that is specifically examined for detecting the characteristic Raman shift for diamond. This broad band laser induced luminescence is falsely identified as a Raman shifted response and thus other mineral particles can be falsely identified and selected as diamond. Additionally, as this primary sorter is a high-mass sorter, the diamond and gangue mixture is fed on a wide belt and the diamond selecting means may select the particles sitting next to the fluorescing particle on the belt as well. Thus the machine can only produce a diamond/luminescing mineral/gangue concentrate. Traditionally this concentrate is hand sorted which is laborious, and as some of the minerals are fairly similar in appearance to the diamonds, can be difficult and unreliable.
In addition to the primary sort achieved by this type of machine, it is necessary to provide a machine for achieving a more accurate sort on the concentrate produced by the process, and remove the other non-diamond material collected in the primary process.
Methods of classifying diamonds by studying them with radiation are generally known (for example EP-A-0345949 as above) but it is desirable to provide a method of classifying diamonds and other objects using radiation in which the problem of the varying trajectories obtained with objects of different sizes and shapes without expensive object projecting means is avoided.